Instant-on heater

ABSTRACT

A heater, used in a Liquid Crystal Display (LCD) pixel array that uses a common voltage polysilicon line to supply heat to the pixel elements, instead of using active control transistor input lines, such as gate input lines. The approach permits the display to be activated during the warm-up process.

RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No.60/790,060, filed on Apr. 7, 2006 and also the benefit of U.S.Provisional Application No. 60/792,897, filed on Apr. 18, 2006. Theentire teachings of the above applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

A typical Liquid Crystal Display (LCD) cannot operate properly at lowtemperature due to liquid crystal (LC) characteristics. For an LCD to bevisible, some type of heating system to warm and maintain a certaintemperature is required.

Two approaches to heating and maintain proper LC temperature have beenemployed in existing LCD technologies. One is an external heater whichis attached to the Thin Film Transistor/Indium Tin Oxide TFT/ITO coverglass following display fabrication. This approach provides maintenanceheating by conducting current through the LCD cover glass. Because itoccupies the inactive area surrounding the active LCD matrix andprovides heat along the edges of the LCD and through the coverglass(es), an external heater is generally inefficient and unable torapidly warm the display during cold start conditions.

Another approach is a row-line heater which is an internal heaterintegrated into the active matrix architecture of the display. The rowline-heater can be used during cold start conditions to rapidly warm theliquid crystal material. The row-line heater is located within pixelarray, very close to LC, so that it provides high efficiency, uniformheating inside of LCD glass.

FIG. 1 is an example of a row-line heater known in the prior art. Thisapproach uses polysilicon row lines that also control the pixeltransistors' input gates. Current can flow through the polysilicon rowlines 10 to produce heat close to the pixel elements 20, and hence closeto the LC material, but during the heating phase, no image can be shownwith this approach, since the row transistors 30 and 32 are saturatingthe row lines 20 to heat them. C_(LC) is the capacitance between theelectrode 25 and an ITO common plate with the liquid crystal in between,and Cstg is the pixel storage capacitance. The warm-up time neededduring cold start of the devices limits their usage in variousapplications of even this design.

SUMMARY OF THE INVENTION

The present invention is an alternative approach that provides rapidheating of LC material even while the display panel is operating. Aheater circuit is fabricated along with the display circuit but does notinterfere with the operation of the display circuits. In particular, thedisplay heater uses a common voltage conductive line to supply heat tothe display panel, instead of active control transistor input lines,such as gate input lines.

No additional or new process steps, beyond those needed to fabricate theLCD itself, are required for fabrication of a heater according to thepresent invention. The internal heater can eliminate the need for anexternal heater, resulting in cost and power reductions. Heateroperation also does not affect the operation of the display panel, sincethe heater is now independent of the display controls. It can,therefore, be used for temperature maintenance and control whilesimultaneously presenting full-rate video imagery.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 is a schematic diagram of a display element heater according tothe prior art that uses the conductive (polysilicon) row lines that alsocontrol the transistor gates to supply heat to the display elements;

FIG. 2 is a schematic diagram of a display element heater according toone embodiment of the invention that instead uses a common voltagepolysilicon line to supply heat to the display elements;

FIG. 3 is a cross sectional diagram of portions of a display element;

FIG. 4 is a top plan view of a display element array;

FIG. 5 is a circuit diagram of a display element array which may use theheater;

FIG. 6 is a circuit diagram of a low power shift register that can beused with the row heater;

FIG. 7 is a cross sectional view of a liquid crystal display using ThinFilm Transistors (TFTs) formed from a single crystal thin film, that canbe used with the heater; and

FIG. 8 is a cross sectional view of another single crystal filmembodiment.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

In the present invention, a polysilicon heater is fabricated in thedisplay active matrix in such a way that it can provide heat, evenduring normal (active) display operation. The instant-on heater linesare formed by modifying the polysilicon structures that previouslyformed the fixed VCOM pixel capacitor electrodes.

FIG. 2 is a circuit diagram of one embodiment. Here two heater structureterminals, at nodes V1 (40) and V2 (42), are used to control the currentthrough the conductive lines. By controlling the DC voltage between nodeV1 and node V2, current through the conductive line(s) 44 produces heatrapidly and physically close to the LC structure in the pixel elements48. Since these conductive lines work as one plate of pixel capacitorwhich stores AC video signals, the DC voltage on the plate has no effecton the respective pixel 48 voltage. Thus, heat can be provided evenwhile the pixel 48 is in active display mode.

On the other hand, the active layer which resides between these heaterlines and LC material shields LC from being affected by DC levels. FIG.3 is a cross sectional view of one of the pixel elements showing this inmore detail. As long as the DC voltages on V1 and V2 are not changeddramatically, the LCD can therefore present an image whilesimultaneously performing temperature warm-up or maintenance.

The heater structure terminals V1 (40)and V2 (42) are preferably broughtout external to the display circuits package so that the heater can becontrolled separately from the operation of the display circuits.

This approach is attractive because it represents an integrated highefficiency approach that can be operated relatively independent of thedisplay function. A higher heater power can be applied during the coldstart for effective warm-up and then be gradually lowered down while tomaintain the desired operating temperature.

In one embodiment, the dimensions for various structures shown in FIG. 3can be glass (50) plus ITO (52) thickness equal to about 0.7 mm; LC (54)about 2.4 um; Active (56) about 320 nm; Electrode (58) about 12 nm; GateOxide (60) about 60 nm; and Heater Line (62) about 500 nm. Otherconfigurations are possible, however.

Because it is physically close to the LC material and operatessimultaneously with display imaging, the instant-on heater eliminatesthe shortcomings of the prior art. It also avoids the heat conductionproblems of external heaters and minimizes the impacts of physicalmounting. The instant-on heater is fabricated during a normal ICprocess. The heater can be configured to achieve the desired resistanceand power independent of the display circuit.

FIG. 4 is a plan diagram showing the layout of a color pixel arrayincorporating the invention.

FIG. 5 is a higher level circuit diagram of a display that myincorporate the heater of FIGS. 2 and 3. As in those figures, nodes Vland V2 provide heating to pixel elements 48. An example of one of thepixel elements 48 is also seen to be controlled by one of the row selectlines 60 and one of the column select lines 62. The row select line 60and column select line 62 are fed from respective ones of a row selectshift register 64 and column select shift register 66.

One or both of the shift registers 64, 66 may be implemented using theapproach described in a co-pending U.S. Provisional Patent ApplicationNo. 60/860,059, filed Nov. 20, 2006, entitled “Shift Register For LowPower Consumption,” which is hereby incorporated by reference in itsentirety. In particular, individual stages of the shift register operatewith a low voltage swing clock signal, with the stage circuit having asingle node driven directly by a single transistor. As shown in FIG. 6(which corresponds to FIG. 3 of the referenced co-pending application)the clock ck drives a stage transistor MP1. The gate of MP1 is fed froma pair of cascode transistors MP2 and MP3 that set the state at node aas determined by inputs e* and vgp. The inverted input from the previousstage e* is fed to the input terminal of inverter INV3 to control thegate of transistor MP3. The MP3 drain terminal controls the gate oftransistor MP1. The source terminal of transistor MP2 is fed fromvoltage VDD.

The pre-charge input pc* is fed through the single NAND gate togetherwith a reset signal r*. The output of the NAND gate drives the gateterminal of signal buffer transistor MN1. The first inverter INV1 andsecond inverter INV2 provide, respectively, the inverted outputs out*and non-inverted output out.

In still other embodiments, the LCD array may, employ single crystalThin Film Transistors (TFTs) to form the display elements 48, of thetype described in co-pending U.S. Provisional Patent Application No.60/838,014, filed Aug. 16, 2006, entitled “Display System With SingleCrystal SI Thin Film Transistors,” the entire teachings of which arehereby incorporated by reference.

FIG. 7 is a schematic cross-sectional view of one embodiment of such aliquid crystal display 110 which includes a transparent substrate 112,such as glass, fused silica or sapphire. Over transparent substrate 112,single crystal Si TFTs 114 are positioned. Also shown are single crystalSi layer 116, gate 118, source 120 and drain 122. Single crystal Si TFTs114 have a thickness in a range of between about 100 nm and about 200nm, preferable in a range of between about 150 nm and about 200 nm. Asused herein, the term “thickness of an array of single crystal Sithin-film transistor” means thickness “a” of single crystal layer 116,as shown in FIG. 7. Insulating material 115 is surrounding singlecrystal Si TFTs 114. Pixel electrodes 124 are positioned over singlecrystal Si TFTs 114. Liquid crystal layer 126 is positioned betweencommon electrode 128 and pixel electrodes 124. Over common electrode 128is positioned transparent substrate 130. Each single crystal Si TFT 114and each pixel electrode 124 are in electrical communication with eachother via metal contact 132. Insulating layer 134 is positioned aroundmetal contact 132. In display 110, transparent substrate 112 and singlecrystal Si TFTs 114 are partitioned by bonding layer 136.

FIG. 8 is a schematic cross-sectional view of another embodiment ofliquid crystal displays of the invention. The display 150 includestransparent substrate 112; and single crystal Si TFTs 114 overtransparent substrate 112. Insulating material 115 is surrounding singlecrystal Si TFTs 114. Pixel electrodes 124 are positioned over singlecrystal Si TFTs 114. Liquid crystal layer 126 is positioned betweencommon electrode 128 and pixel electrodes 124. Over common electrode 128is positioned transparent substrate 130. Each single crystal Si TFT 114and each pixel electrode 124 are in electrical communication with eachother via metal contact 132. Insulating layer 115 is around metalcontact 132. In display 150, single crystal Si TFTs 114 are partitionedon transparent substrate 112.

A display according to the present invention may be used in a variety ofapparatus, including night vision devices, digital cameras, (Single LensReflectors (SLR) or video cams), handheld video games, and others.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. An apparatus comprising: a plurality of display elements, with each element comprising at least a pixel and a transistor, with the transistor having at least a gate terminal, and the transistor arranged to control an operating state of the pixel; a common voltage line, the common voltage line connected to two or more display elements at a point other than the gate terminal, the common voltage line also having two terminals to provide a first node and a second node; and a control circuit, for controlling voltages provided to both the first node and second node, such that a voltage difference between the first and second nodes causes current to flow through the common voltage line, and to thereby heat the display elements connected thereto.
 2. An apparatus as in claim 1 wherein the display elements are arranged in a matrix of rows and columns.
 3. An apparatus as in claim 2 additionally comprising at least one column select line and one row select line.
 4. An apparatus as in claim 3 wherein the common voltage line is independent of the column select line.
 5. An apparatus as in claim 1 wherein the common voltage line is disposed adjacent to one of the transistor and pixel element in each display element.
 6. An apparatus as in claim 1 wherein the common voltage line is located in a plane beneath an active layer of the pixel elements.
 7. An apparatus as in claim 1 wherein the common voltage line is not coupled to a gate control input of the transistors.
 8. An apparatus as in claim 3 wherein the common voltage line is independent of the row select line.
 9. An apparatus as in claim 1 wherein the display elements are used in one of a digital camera, digital Single Lens Reflex (SLR) camera, night vision display, handheld video game mobile telephone, or video eyewear device.
 10. An apparatus as in claim 3 wherein at least one of the row select line or column select line is provided from a low power shift register.
 11. An apparatus as in claim 10 wherein the low power shift register comprises a stage circuit having a single voltage node driven by a single transistor. 